A spine includes a series of joints or motion segments. The components of each motion segment include two adjacent vertebrae, their apophyseal joints, an intervertebral disc, and connecting ligamentous tissue. Each motion segment is capable of flexion, extension, lateral bending, and translation. Each component of the motion segment contributes to these capabilities and to the mechanical stability of the spine.
The intervertebral disc is one component that facilitates spine motion by allowing slight relative movement between adjacent intervertebral discs as well as holding the vertebrae together. The discs comprise an outer annulus fibrosus which surrounds and contains a nucleus pulposus. The nucleus acts as a shock absorber and a spacer to separate adjacent vertebra. In a healthy spine, the motion segments, including the discs, collectively enable the familiar kinematics of the spinal column. However, degeneration of the disc can cause great and sometimes debilitating pain.
For example, radicular pain in the lower extremities is often a symptom of a herniated disc. A herniated disc is characterized by rupture of or tear in the annulus fibrosus which permits a portion of the nucleus to extrude therefrom. If the nucleus extrudes in proximity to the numerous nerves surrounding the spine, the pressure, or the mere contact, of the nucleus on the nerves may cause severe pain. In addition, axial pain is often a symptom of degenerative disc disease. Degenerative disc disease is generally associated with dehydration of the nucleus that occurs with age. When the nucleus dehydrates, it loses its ability to absorb shock, which may lead to axial pain.
Treatment methods for repair of disc disorders include spinal fusion. One type of spinal fusion procedure requires resection of a portion of the disc. The procedure for removing a portion of the disc is known as a discectomy. Once a portion of the disc is removed, another material or device is inserted into the space created to stabilize the spinal column. There are a variety of devices available for insertion into the disc space. For example, one fusion procedure includes placing a cage between and in contact with the vertebra and packing the cage with graft material. The graft material may bond with the endplates of the adjacent vertebra thus fusing the vertebra together. However, these types of devices require significant retraction of tissue to allow the surgeon sufficient access to the disc and to insert the device into the corresponding disc space. Consequently, patient recovery time may be significant simply because of the invasiveness of these procedures.
Thus, devices and methods for spinal fusion that are stable, yet minimally invasive, are needed.